1. Field of the Invention
The present invention relates generally to RF devices, and particularly to relatively high power RF devices with improved thermal characteristics.
2. Technical Background
The primary use of some mobile phones (e.g., smartphones) is data communications rather than voice communications. More and more of the public is using their mobile telephones for Internet access, social media access and for downloading videos and pictures from both of these sources. Thus, the demand for mobile data services is growing at an exponential rate. This necessarily means that the bandwidth provided by the telecommunications infrastructure (as well as the mobile telephone handset itself) must continue to grow if it is to keep pace with the demand. In order to accomplish this, new mobile telephone handsets and new base station equipment must be configured to operate over multiple frequency bands and communications systems (e.g., Wi-Fi, 3G, 4G, Bluetooth, etc.). Stated differently, new mobile telephones and base stations must provide more functionality in the same (or less) space. Because radio frequency (RF) devices such as power amplifiers, couplers, baluns, etc., are integral parts of every wireless device or system, they must also do more in a smaller footprint. Accordingly, size reduction, improved heat dissipation, power handling, and cost reduction are important considerations for today's RF component designs.
Referring to FIG. 1, a sectional view of a conventional softboard RF device 1 is shown. The RF device 1 includes three softboard layers (1, 2, and 3) that are laminated together to form an integrated device. Before lamination is performed, conductive traces 4 are disposed on the middle softboard layer 2 by way of conventional photolithographic methods. One of the advantages of using all-softboard related devices relates to their excellent electrical performance. One reason for this is that softboard devices use photolithographic processes for circuit trace fabrication; photolithography produces excellent conductive traces (i.e., high purity copper traces that have an excellent shape) that exhibit low conductor resistivity (e.g., Cu-1.7 μΩ-cm; Ag-1.55 μΩ-cm; and Au-2.2 μΩ-cm). Moreover, softboard devices are easily configured to match the coefficient of thermal expansion (CTE) of the SMT carrier printed circuit board (PCB). As a result, the reliability of softboard devices is typically excellent. Finally, softboard devices are amenable to being manufactured in large batches, which typically results in lower costs.
On the other hand, the maximum power handling ability of softboard based RF circuits is relatively low because they are characterized by relatively poor thermal conductivity and poor Maximum Operating Temperature (MOT). Power Handing is a function of thermal conductivity, electrical performance, circuit geometry, MOT and part size. MOT is a function of material properties. A typical glass-epoxy laminate printed circuit board (PCB), such as FR4, has a MOT of 130° C. Teflon based PCB materials have a MOT up to 280° C. Polyamides have a MOT>200° C. Copper has an MOT of 130° C. in air. The material with the lowest MOT, of course, sets the limit for the entire system. Standard softboard dielectrics thus have a relatively low thermal conductivity of about 0.25 W/mK up to 1 W/mK, with doped softboard dielectrics reaching up to about 3 W/mK.
Referring to the heat distribution pattern 5 shown in FIG. 1, the amount of heat that is laterally conducted by softboard type devices is rather limited. Those skilled in the art assume that the thermal energy generated by all-softboard devices dissipates at about a 45° angle through the softboard dielectric materials. Thus, the thermal distribution pattern 5 is limited to a small region on each major surface of the device 1. Accordingly, one drawback to this approach is that the thermal energy is dissipated over a rather small surface area, and as a result, the thermal energy builds up over time and becomes problematic. Stated differently, the amount of power that a device can handle is limited by its heat dissipation capabilities. All-softboard devices typically exhibit a low maximum continuous operating temperature (e.g., about 120° C. to 200° C. typical). The glass transition temperature (Tg) is also relatively low (e.g., 75° C. to 150° C. being typical values for standard PCB materials).
Referring to FIG. 2, a top view of the conventional softboard RF device depicted in FIG. 1 is shown. This view represents a typical configuration that provides three edge vias 6 on each side thereof, with one edge via 6 disposed on each end. Using the dimensions in the illustration, and assuming 1 mil thick copper, the surface area provided by each via is about 0.05 mm2. The combined area (for three vias) is about 0.149 mm2. Even if one were able to direct the thermal energy to the edge of the device, there is only a small surface area available for heat dissipation.
Thus, conventional stripline (or microstrip) designs can be implemented using softboard materials that are disposed in a relatively small package. While these devices offer some convenience, they exhibit power handling limitations (e.g., up to 100-200 W) that are due to thermal resistance and maximum operating temperature (MOT) limitations.
Instead of using softboard dielectric layers, some manufacturers produce RF circuits (e.g., couplers) using low temperature co-fired ceramic (LTCC) or high temperature co-fired ceramic (HTCC) ceramic technology. Some of the advantages of ceramic devices include: high thermal conductivity (30 W/m° C. to 170 W/m° C. typical); low thickness variations (when lapped +/−0.0005 typical); high maximum continuous operating temperatures (500° C. to 2000° C. typical); and a high glass transition temperature (Tg). On the other hand, the maximum power handling of ceramic RF devices is limited by electrical performance (where insertion loss is typically higher than similar PCB devices) and solder joint failure (due to reflow or cracked solder joint) at the device-to-carrier PCB interface (solder joint reflow often leads to failed solder joints). In the solder reflow failure mode, the RF power flowing through the device can cause an all-ceramic RF device to become so hot that the solder reflows and the device separates from the carrier PCB. Thus, the high thermal conductivity of conventional ceramic devices can lead to device failure if the power level is not limited. The cracked solder joint failure mode is typically due to the CTE mismatch between the all-ceramic RF device and the softboard carrier PCB (Ceramic exhibits a CTE of about 5 ppm/° C. typical whereas the carrier PCB exhibits a CTE of about 15 to 25 ppm/° C. typical in x/y axis).
In addition, all-ceramic RF devices have a relatively low RF performance (vis á vis softboard devices) due to the conductor material and conductor shape tolerance. Thus, RF circuits based on either all-softboard or all-ceramic constructions have inherent features which limit their maximum high power rating and reliability. Moreover, when an RF device is used in a surface mount technology (SMT) assembly, the device is mounted on a carrier PCB and the thermal path is further degraded by the carrier PCB.
Referring to FIG. 3, a cross-sectional view of a conventional LTCC or HTCC (both all-ceramic) device 1 is shown. The ceramic device 1 includes three ceramic layers (2, 3 and 4) with traces 5 formed on the center ceramic layer 3. In this view, the cross-sectional shape of the conductor 5 is of interest. A conventional all-ceramic RF device 1 typically employs a process whereby the circuit traces are screen printed. In one approach, a thick film conductive paste is disposed on a ceramic substrate to implement the conductive portions of the circuits. The conductive traces 5 exhibit high resistivity due to porosity and impurities. The high resistivity of the conductors 5 is a result of the materials used in the screen printing process; i.e., the conductors 5 are made from an Ag or a W (5.7 microohm-cm) paste. Moreover, the cross-sectional shape of the conductive traces 5 is not conducive to high RF performance. In another conventional approach, a thin film sputtering layer is disposed on ceramic to implement the circuitry, which eliminate the above issue, however the thickness of the conductors are very thin, and not conducive to high power handling. Accordingly, the electrical performance of ceramic RF devices is relatively poor in comparison to softboard devices. Finally, the size of the footprint of a panel of ceramic devices (before singulation) is relatively small (e.g., about 4″×4″ to 6″×6″) as compared to softboard panels (which may be 12″×18″ to 18″×24″ before singulation). The thick-film approach also yields relatively large line width tolerances unless secondary etching processes are employed. Obviously, each additional process step that must be included in the overall process results in additional costs.
FIG. 4 represents a hybrid approach that attempts to take advantage of the benefits associated with sailboard and ceramic devices, while eliminating or mitigating their disadvantages. Specifically, FIG. 4 is a detail view that shows a four port coupler component 1000 that includes two planar transmission lines 1142 and 1144 disposed between two ground planes 1020. The transmission lines 1142 and 1144 are formed on the upper and lower surfaces, respectively, of the inner dielectric layer 1146. Disposed between transmission line conductors 1142, 1144 and ground planes 1020 are the upper dielectric layer 1012 and lower ceramic puck 1160, respectively. The puck is disposed in a pocket 1016-1 formed in softboard layer 1016. The dielectric layers 1012, 1016, and 1146 are formed using any suitable type of circuit board material, e.g., FR-4, PYRALUX®, ROGERS RO3003®, etc. Layer 1146 includes transmission lines 1142 and 1144 disposed on either side thereof. The transmission lines 1142 and 1144 are formed using photolithographic techniques. The vias (e.g. 1200, 1120, 1148 and 1600) are formed using plated through holes (PTH) to provide interconnections between internal metal layers and internal metal layers to external metal layers.
While the approach considered in FIG. 4 may represent an improvement over softboard devices (with respect to power handling), there are also drawbacks that must be considered. In this approach, each individual device 1000 requires a small individual ceramic puck 1160 placed in pockets 1016-1 for each individual device. One drawback to this approach is, therefore, that each puck cavity requires tight positional tolerance for the ceramic puck insert. In addition, the conductors 1142 and 1144 are used to realize a stripline configuration disposed between two softboard layers 1012 and 1016. Accordingly, the thermal path is somewhat degraded by the two softboards (1012, 1016). Another drawback relates to the additional process steps required to implement the devices. For example, each device requires plated through-hole interconnections between internal metal layers to external metal layers. In addition, the process uses bonding layers 1018 between certain layers.
Having completed a brief survey of conventional RF techniques, the discussion now turns to the environment in which each of these devices (i.e., softboard, ceramic and hybrid composite) are typically used. To be specific, the conventional devices described above are typically mounted on an end-user's printed circuit board and interfaced to the end-user's control circuitry, amplifier and/or load. Often, conventional power amplifiers are fabricated using SMT plastic over-molded transistors that consist of a transistor die mounted to a copper slug (for heat dissipation purposes); these elements are covered by a plastic over-mold. The amplifier is mounted on a carrier PCB or in a cutout of a PCB (for improved heat dissipation). In the case of a flip chip amplifier, the amplifier is mounted on a PCB (or ceramic board) that is subsequently mounted on a copper plate that functions as a heat sink. In both cases, the transistor I/O pads must be connected to the corresponding PCB pads by solder connections. Moreover, one should not forget that the amplifier transistors have input and output impedances that are significantly lower than the 50 Ohm system impedance. Thus, both input and output impedance matching networks are usually required. Briefly stating the above, the interconnections and the matching networks take up too much end-application circuit board “real estate.”
Based on the drawbacks outlined above, what is needed is an improved RF device that provides relatively low insertion loss (IL) and low thermal stress, while exhibiting relatively high thermal conductivity. An RF device is needed that provides high power handling capability and low manufacturing cost. What is also needed is an RF device that exhibits “end application” flexibility. Stated differently, an RF device that can be mounted in a variety of end-application environments to eliminate non-value added interfaces is very desirable. As those skilled in the art will appreciate, the elimination of duplicative interfaces and interconnections could go a long way toward reducing the size and cost of RF assemblies while improving their overall performance.